1. Field of the Invention
The present invention relates to a laser apparatus in which a plurality of laser diodes are fixedly arranged on a block, and a laser apparatus in which laser beams emitted from a plurality of laser diodes are collimated into a plurality of collimated laser beams which are parallel to each other.
The present invention also relates to a method for producing a laser apparatus in which laser beams emitted from a plurality of laser diodes are collimated into a plurality of collimated laser beams which are parallel to each other.
The present invention further relates to a fiber module comprising a light source and an optical fiber which light emitted from the light source enter.
2. Description of the Related Art
Conventionally, in order to generate a laser beam having a ultraviolet wavelength, wavelength conversion lasers, excimer lasers, and Ar lasers are used. In the wavelength conversion lasers, infrared light emitted from a solid-state laser excited with a semiconductor laser is converted into a third harmonic having an ultraviolet wavelength.
Further, recently, GaN-based compound semiconductor lasers (laser diodes) which emit a laser beam having a wavelength in the vicinity of 400 nm have been provided, for example, as disclosed in Japanese Journal of Applied Physics Part 2 Letters, vol. 37, 1998, pp. L1020.
Light sources which emit laser beams having the wavelengths as mentioned above are considered to be used in exposure systems for exposure of photosensitive materials which are sensitive to light in a predetermined wavelength range including an ultraviolet wavelength range of 350 to 420 nm. In such a case, the light sources for exposure are required to have sufficient output power for exposing the photosensitive materials. The above predetermined wavelength range is hereinafter referred to as the ultraviolet range.
However, the excimer lasers are large in size, and the manufacturing costs and maintenance costs of the excimer lasers are high.
In the wavelength conversion lasers which convert infrared light into a third harmonic in the ultraviolet range, the wavelength conversion efficiency is very low. Therefore, it is very difficult to achieve high output power. In a typical wavelength conversion laser at the currently practical level, a solid-state laser medium is excited with a semiconductor laser having an output power of 30 W so as to output a fundamental harmonic having a wavelength of 1,064 nm and an output power of 10 W, the fundamental harmonic is converted into a second harmonic having a wavelength of 532 nm and an output power of 3 W, and a third harmonic having a wavelength of 355 nm (i.e., a sum frequency of the first and second harmonics) and an output power of 1 W is obtained. In this wavelength conversion laser, the efficiency in electric-to-optical conversion in the semiconductor laser is about 50%, and the efficiency in conversion to the ultraviolet light is as low as about 1.7%. In addition, since an optical wavelength conversion element is used in the above wavelength conversion laser, and the optical wavelength conversion element is expensive, the manufacturing cost of the wavelength conversion laser is high.
Further, the efficiency in electric-to-optical conversion in the Ar lasers is as low as 0.005%, and the lifetime thereof is as short as about 1,000 hours.
On the other hand, since it is difficult to obtain a low-dislocation GaN crystal substrate, an attempt has been made to achieve high output power and reliability in a GaN-based compound semiconductor laser. In the attempt, a low-dislocation region having a width of about 5 micrometers is produced by a growth method called ELOG (epitaxial lateral overgrowth), and a laser region is formed on the low-dislocation region. However, even in the attempt, it is difficult to obtain a low-dislocation substrate having a large area. Therefore, no GaN-based compound semiconductor laser having a high output power of 500 mW to 1 W has yet been commercialized.
In another attempt to increase output power of a semiconductor laser, for example, it has been considered to form a hundred cavities each of which outputs light with 100 mW so as to obtain a total output power of 10 W. However, it is almost unrealistic to manufacture as many as 100 cavities with high yield. In particular, it is difficult to manufacture GaN-based compound semiconductor lasers each having many cavities since manufacture of GaN-based compound semiconductor lasers with a high yield of 99% or greater is difficult even when the GaN-based compound semiconductor lasers each have a single cavity.
In view of the above circumstances, the present inventors have proposed laser apparatuses having particularly high output power (which are also referred to as optically-multiplexing laser-light sources), as disclosed in U.S. Patent Applications 20020090172 A1 and 20030048819 A1 (respectively corresponding to Japanese Unexamined Patent Applications Nos. 2001-273849 and 2001-273870).
A laser apparatus disclosed in U.S. Patent Application 20020090172 A1 is constituted by a plurality of laser diodes, a single multimode optical fiber, and an optical condensing system which collects laser beams emitted from the plurality of laser diodes, and couples the collected laser beams to the multimode optical fiber. This laser apparatus can be manufactured at low cost. In a preferred embodiment of the laser apparatus, the plurality of laser diodes are arranged so that light-emission points of the laser diodes are aligned along a certain direction.
On the other hand, in a laser apparatus disclosed in U.S. Patent Application 20030048819 A1, a plurality of multicavity laser-diode chips each having a plurality of light-emission points are fixedly arranged.
When a plurality of laser diodes are arranged so that the light-emission points are aligned along a certain direction, normally, the plurality of laser diodes are fixed to a block such as a heat dissipation block made of copper or copper alloy.
The above laser apparatuses have the following problems (1) to (3).
(1) Since the laser beams emitted from each laser diode are divergent, it is necessary to collimate the divergent laser beams through collimator lenses, and make the laser beams converge on a point. At this time, the collimator lenses may be separately arranged, or integrally formed into a collimator-lens array in which collimator-lens portions are arranged along a line. In either case, it is necessary to accurately position the laser diodes and the collimator lenses or the collimator-lens array so that the optical axes of the collimator lenses (or the collimator-lens portions) respectively coincide with the light-emission axes of the laser diodes. When the above positioning is inaccurately performed, it is impossible to make the plurality of laser beams converge on a small spot. Therefore, for example, when a photosensitive material is exposed to the laser beams in order to form an image, it becomes impossible to form a fine image by the exposure.
(2) In order to make divergent laser beams emitted from a plurality of laser diodes converge on a point, it is necessary to collimate the divergent laser beams through collimator lenses, and make the collimated laser beams propagate parallel to each other and enter a condensing lens. In order to realize this operation, the laser diodes and the collimator lenses are required to be accurately positioned so that the focal points of the collimator lenses coincide with the light-emission points of the laser diodes, and the lines each passing through one of the light-emission points and a center of a corresponding one of the collimator lenses are parallel to each other. If the laser diodes and the collimator lenses are inaccurately positioned, it becomes impossible to make the plurality of laser beams converge on a sufficiently small spot.
In order to prevent occurrence of the above problem, it is necessary to position the collimator lenses and the laser diodes with a small pitch and micron to submicron alignment precision in the X, Y, and Z directions. When the pitch is small, the gaps between the collimator lenses become small, for example, as small as 100 micrometers or less. Although a method for adjusting the position of each lens during activation of the laser diodes is known, it is not easy to adjust the position of each lens by moving the lens in the X, Y, and Z directions relative to a corresponding laser diode, since there is no sufficient space for equipment which securely holds the lenses.
For example, in order to adjust the three-axis alignment, it is necessary to fix each collimator lens to a laser block through a holder element. Therefore, fixation at least two places is required for each collimator lens. Therefore, when seven laser diodes are used for optical multiplex, bonding is required to be performed at fourteen places with micron to submicron alignment precision in the X, Y, and Z directions. At this time, the total yield is proportional to the fourteenth power of the fixation yield at each place of fixation. Therefore, even in the case where the reliability of fixation at each place of fixation is increased, it is very difficult to achieve a satisfactory yield when the number of places of fixation increases.
In addition, it is important to make bonding surfaces of the holder element, the lenses, and the laser diode block parallel to each other. However, due to inaccuracy of the individual parts, the parallelism between the holder element, the lenses, and the laser diode block is not necessarily compatible with the alignment between optical axes of the lenses and the laser diodes, and therefore it is difficult to ensure precision. Thus, the alignment yield decreases, alignment time and parts cost increase, and the total cost of the laser apparatus also increases.
(3) In many optically-multiplexing laser-light sources using a multimode optical fiber, a plurality of laser diodes, an optical condensing system, and an end portion of the optical fiber are contained in a package so as to form a fiber module. In such a fiber module, the end portion of the optical fiber is fixed to a fiber holder, a bracket, or the like which is internally fixed to the package. Conventionally, the optical fiber is fixed by YAG welding or brazing of a ferruled optical fiber.
However, the precision of the fixation of the optical fiber by the YAG welding is plus/minus 1 to plus/minus 5 micrometers, and the precision of the fixation with the brazing material is plus/minus 5 to plus/minus several tens of micrometers. Therefore, it is impossible to accurately arrange the optical fiber so as to align the optical fiber with a position at which the laser beams converge. Actually, the coupling efficiency of the laser beams to the optical fiber is about 80% in the case of YAG welding, and about 60 to 80% in the case of brazing. Further, an attempt to bond the optical fiber with an adhesive has been made. However, the precision of the conventional fixation of the optical fiber with an adhesive is similar to those in the cases of the YAG welding or brazing.